In the past unwanted noise and vibration has been controlled by muffling or isolation. However, the principle of superposition means that noise and vibration can also be controlled by means of so-called "anti-noise", that is the production of an acoustic signal having the same spectral characteristics as the unwanted noise or vibration but 180.degree. out of phase.
U.S. Pat. No. 4,527,282 discloses a system where a speaker generates a cancelling acoustic signal which is mixed with an unwanted acoustic signal. A microphone senses the residual acoustic signal which is then amplified and inverted to drive the speaker. Systems of this type are prone to instabilities and are restricted in the range of frequencies over which they are effective.
A system which avoids the instability problems of simple systems, such as that disclosed in U.S. Pat. No. 4,527,282, is described in U.S. Pat. No. 4,490,841. In the described system, the residual signal is analysed by means of a fourier transformer. The resultant fourier coefficients are then processed to produce a set of fourier coefficients which are then used to generate a cancelling signal.
Systems which process signals in the frequency domain, following fourier transformation, perform their function well under steady-state conditions. However, if the fundamental frequency of the noise signal changes, the system requires several cycles to re-establish effective cancellation. This is due to the time taken to perform the fourier transformation. If such apparatus is used in an internal combustion engine noise control system, bursts of noise will occur during acceleration and deceleration. These bursts may, in fact, have a higher peak value than the unsuppressed steady-state engine noise. Furthermore, the need to carry out high-speed digital signal processing means that these systems are expensive to implement.